1. Field of the Invention
The present invention relates to electronics and electrical systems. More specifically, the present invention relates to analog to digital converters.
2. Description of the Related Art
Analog to digital converters are widely used for converting analog signals to corresponding digital signals for many electronic circuits. For example, a high-resolution analog to digital converter (ADC) may find application in radar, missile, and communications systems. There are two basic techniques for performing analog to digital conversion: an open-loop technique and a feedback technique. An open-loop ADC generates a digital signal directly in response to an analog input signal. This approach uses precisely matched components (such as resistors and capacitors) to digitize the input signal. The resolution and accuracy of an open-loop ADC depend on a precise matching of these components. However, highly precise components are difficult to achieve using conventional integrated circuit processing techniques.
A Delta-Sigma (ΔΣ) ADC (also known as a Sigma-Delta ADC) is a feedback type ADC that subtracts a feedback signal from the analog input signal to provide an error signal. The error signal is filtered and then quantized to form a digital output signal. The Delta-Sigma approach achieves high resolution by precise timing instead of by precisely matching components as in open-loop converters. The Delta-Sigma technique is therefore the preferred technique for many applications.
A Delta-Sigma ADC typically includes a Delta-Sigma modulator and a digital filter. The Delta-Sigma modulator (also known as a Sigma-Delta modulator) uses oversampling (i.e., a sampling rate above the Nyquist rate) and filtering to develop a high signal-to-noise ratio in the signal band. The digital filter then attenuates the out-of-band quantization noise and decimates the signal to provide an N-bit data word at the Nyquist rate.
A simple Delta-Sigma modulator includes a quantizer, an analog filter, and a digital to analog converter (DAC). The quantizer generates a digital output signal in response to the filtered difference between the analog input signal and a feedback signal. The feedback signal is the digital output signal reconverted to analog by the DAC. The analog filter shapes the quantization noise to be higher at frequencies outside of the signal band. The ADC digital filter typically has a lowpass characteristic with a cutoff frequency at the Nyquist frequency. Since the sampling frequency is much higher than the Nyquist frequency, the filter can usually attenuate this out-of-band quantization noise sufficiently.
High resolution and low distortion can be achieved by using a single-bit quantizer inside the Delta-Sigma modulator. However, single-bit Delta-Sigma ADCs are less stable, suffer from more quantization noise, and require a higher oversampling ratio (OSR, the ratio of the sampling rate to the signal bandwidth). By replacing the single-bit quantizer with a multi-bit ADC inside the loop, the Delta-Sigma ADC can use a lower sampling rate to achieve the same resolution with much better stability. However, due to the mismatch of elements inside the feedback DAC, multi-bit Delta-Sigma ADCs suffer from the non-linear distortion of the feedback DAC, which leads to poor resolution and high distortion in the overall ADC output. The most commonly used solution to this problem is to randomize the mismatch by dynamically swapping the elements inside the feedback DAC using a noise-shaped DAC.
The noise-shaped DAC approach inserts an extra noise-shaping circuit between the internal ADC and the feedback DAC in a multi-bit Delta-Sigma modulator to randomize the connections between the ADC and the DAC to spectrally shape the conversion noise generated by the mismatched components. The conversion noise can then be removed along with the quantization noise by the digital filter of the Delta-Sigma ADC. This noise-shaped DAC solution can minimize the non-linear distortion and in-band noise very effectively. However, the extra noise-shaping circuit which is inserted between the internal ADC and the feedback DAC causes a delay and therefore degrades the overall ADC performance. With the extra loop delay caused by the noise-shaping circuit, the Delta-Sigma ADC becomes more unstable. It requires effort to stabilize the system at the expense of lower overall resolution.
Hence, a need exists in the art for an improved system or method for reducing noise due to mismatch errors in a multi-bit Delta-Sigma ADC.